In the last two decades there has been extensive development of plasma arc spraying and many applications have been developed. Plasma spraying offers the ability to create coatings and free standing structures of virtually any material which can be melted.
Of particular interest has been the adhering of ceramic surfaces to metal elements, to protect them from thermal and abrasive environments. As is well known, substantial problems of incorporating ceramic material with metal structures arise from the differences in thermal expansion which exist between most ceramics and most metals. High temperature structures generally utilize high temperature metals, such as superalloys of iron, nickel, and cobalt. These materials characteristically have high thermal expansion coefficients of the order of 10-14.times.10.sup.-6 per .degree.C. The ceramics which are of most interest tend to be those containing alumina, zirconia, magnesia, and like materials which have low thermal expansion coefficients, of the order of 5-10.times.10.sup.-6 per .degree.C.
Several different approaches have been utilized to obtain good adhesion between a low expansion ceramic structure and a high expansion metal structure. One approach has been to form sprayed composite interlayers by mixing metal and ceramic powders to provide a gradation in composition, starting with entirely metal powder at metal surface, progressing through partial metal and partial ceramic, and ending with entirely ceramic. Still another method described in U. S. Pat. No. 4,273,824 of McComas et al., having common assignee herewith, has been to first adhere a fiber metal mat to a metal surface, by brazing or diffusion bonding. Plasma spraying is used to build up a coating of ceramic on the fiber mat. To improve bonding of the ceramic to the fiber mat, a thin bond coating of a metal has been first sprayed on the mat. While sucess has been met with these approaches, there are still improvements needed for lower cost and improved performance.
Plasma spray coatings and free standing plasma sprayed structures, particularly when they are accreted to relatively great thicknesses, tend to be materials which have relatively low strength compared to materials which have been formed by other methods. Thus, it is desirable to find convenient ways to include fibers within a built up plasma sprayed structure since fibers will enhance their strengths. Boron fiber reinforced aluminum composites are one known combination of fibers with plasma coatings. They are made by laying fibers on thin metal foils and spraying with aluminum to bond the fibers to the foil, to form laminae. Subsequently, many such fiber-foil laminae are pressed together to form generally thin and wide articles, such as airfoils. But the process is costly. Also, there is no feasible way of incorporating fibers transverse to the nominal plane of the articles, owing to the mode of construction from laminae.